1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulically operated continuously variable transmission of belt-and pulley type used for a motor vehicle.
2. Discussion of the Prior Art
A belt-and-pulley type continuously variable transmission whose speed ratio is continuously variable is known as a transmission incorporated in a power transmitting system for a motor vehicle. This continuously variable transmission includes a first and a second shaft, a pair of variable-diameter pulleys provided on the first and second shafts, respectively, a belt connecting the pulleys for transmitting power therebetween, and a pair of hydraulic actuators for changing effective diameters of the pulleys. In this transmission, a suitably regulated line pressure of a hydraulic system is applied as a belt tensioning pressure to one of the hydraulic actuators, so as to control the tension of the belt engaging the pulleys. An example of such a belt-and pulley type continuously variable transmission is disclosed in laid-open Publication No. 58-94661 of unexamined Japanese Patent Application. The transmission disclosed in this publication is adapted to receive power from an engine of the vehicle, through a reversing gear device for selecting a forward or a reverse running of the vehicle.
If a shift lever of the vehicle is operated to any one of forward drive positions, such as LOW, SECOND and DRIVE positions, while the vehicle is running in the reverse direction, for example, the direction of the rotary motion transmitted from the reversing gear device to the continuously variable transmission is reversed, whereby the rotating direction of the pulleys is reversed. Consequently, an apparent frictional force between the belt and pulleys of the transmission is reduced, and the belt tends to slip on the pulleys.
Generally, the belt used in the continuously variable transmission has a multiplicity of blocks, which are pressed against walls of the pulleys defining respective V grooves. The power received by the transmission is transmitted through some of these belt blocks which are actually in contact with the pulleys. When the rotating direction of the pulleys is reversed as described above, the continuously variable transmission may be temporarily placed in a condition in which only part of the belt blocks which are currently in contact with the pulleys serves to transmit the torque in the normally selected direction. (Namely, the number of the blocks which serve to transmit the torque in the selected direction is reduced as compared with that of the blocks which are actually in contact with the pulleys.) In this condition, the apparent frictional force between the belt and the pulleys is reduced, and the torque transmitting capacity of the belt is accordingly reduced.